Hitherto, the bumpers of cars have usually been made of a steel sheet. However, steel bumpers are undesirably heavy and have poor impact-relieving property. Accordingly, in order to eliminate the above-mentioned disadvantages, the production of a new type of bumper, exhibiting an enhanced safety and a decreased weight, by using a polyurethane resin in place of the steel sheet has been attempted. However, the lightness in weight of the resultant polyurethane resin bumper was still unsatisfactory and the polyurethane resin, which was expensive, caused the resultant bumper to be undesirably expensive. Therefore, it was desired to find a material useful for making the bumpers having a satisfactory low price and light weight.
Under these circumstances, crystalline ethylene-propylene copolymers were expected to be a material useful for making cheap and light bumpers.
Generally, it is known that conventional crystalline ethylene-propylene block copolymers, adequate for injection molding processes, exhibit an excellent rigidity and satisfactory resistance to thermal deformation and surface hardness. However, it is also known that the conventional crystalline ethylene-propylene block copolymers exhibit a poor lacquer-bonding property and a poor resistance to mechanical impact applied thereto at a low temperature of, for instance, about -30.degree. C. Therefore, it was found that the conventional crystalline ethylene-propylene block copolymers were unsatisfactory as a material for making bumpers having all the properties of excellent rigidity, satisfactory resistance to thermal deformation, resistance to mechanical impact at a low temperature and capable of good lacquer-bonding.
In order to enhance the resistance to mechanical impact at a low temperature and to enhance the lacquer-bonding property of the crystalline ethylene-propylene block copolymers, it was attempted to blend the conventional crystalline ethylene propylene block copolymer with an amorphous ethylene-propylene copolymer alone or a mixture of the amorphous ethylene-propylene copolymer with various types of polyethylene.
However, in this attempt, it was found that the above-mentioned modes of blends caused the resultant copolymer blends to exhibit lower rigidity and resistance to thermal deformation than those of the crystalline ethylene-propylene block copolymers. That is, it failed to provide bumpers having an excellent rigidity and resistance to mechanical impact by using the conventional crystalline ethylene-propylene block copolymers and amorphous ethylene-propylene copolymers.